Higher harmonics of zero sequence current and voltage as a source of information for single-phase earth fault protection.

Single-phase earth faults in networks with ungrounded neutral are common and belong to a wider class of high-impedance damage. The problem of protection against such damage is quite complex and has been actively studied around the world in recent decades. In this article, it is proposed to use the information contained in the high-frequency components of the current and voltage of the zero-sequence during the transient process of insulation damage for targeted protection. The method of selection of high-frequency components is considered. The possibility of using a Petersen coil in the network is considered, as well as the probable deviation from the resonant tuning of the Petersen coil. The peculiarities of the transient process depending on the instantaneous voltage at the moment of a short circuit are investigated. Simulation of single-phase short circuits with different active resistance in the short circuit place is carried out. The spectral composition of zero-sequence currents and voltages during the transient process is analyzed. The dependence of the power of high-frequency components on the quality of Petersen coil tuning, instantaneous voltage at the time of short circuit, and active resistance at the time of short circuit is investigated. The principle of protection is based on determining the direction of the total reactive power of the higher harmonics of currents and voltages of zero sequence and the numerically found derivatives. The results of mathematical modeling showed that at the moment when the phase is closed to the ground, due to the recharging of the capacitance of the phase conductors in currents and voltages of zero sequence, higher harmonics occur. The power of the harmonics from the second to the tenth can be compared with the power of the first harmonic and they should be used for protection. Positive results of work of the offered way of protection at mathematical modeling and at tests on a laboratory stand are received.

Analysis of the Influence of the Insulation Parameters of Medium Voltage Electrical Networks and of the Petersen Coil on the Single-Phase-to-Ground Fault Current

Settings of protection are essential to ensure the sensitivity and selectivity needed to detect defects. Making the correct settings requires the calculation of the fault currents with as little error as possible. Fault currents are influenced by the parameters of the electrical networks, including the state of the insulation and the Petersen coil, which changes during their operation electrical networks. This paper analyzes how the insulation parameters of medium voltage power lines, the parameters of the Petersen coil used to treat the neutral of the medium voltage electrical network and the value of the resistance at the fault location influence the fault current in the case of a single-phase fault. The large number of single-phase faults that occur in medium voltage electrical networks justifies this analysis. The symmetrical components method was used to calculate the fault current. The results obtained by calculation were verified experimentally by causing a single-phase-to-ground fault in a real medium voltage network. The paper presents the situations in which the analytical calculation of the single-phase-to-ground fault current can lead to inadmissibly large errors, even over 50%, but also the situations in which the errors fall below 3%.

Ground Fault Protection methods for Distribution Systems

The system grounding method option has a direct influence on the overall performance of the entire medium voltage network as well as on the ground fault current magnitude. For any kind of grounding systems: ungrounded system, solidly and low impedance grounded and resonant grounded, we can find advantages and disadvantages. A thorough study is necessary to choose the most appropriate grounding protection system. The power distribution utilities justify their choices based on economic and technical criteria, according to the specific characteristics of each distribution network. In this paper we present a medium voltage Portuguese substation case study and a study of neutral system with Petersen coil, isolated neutral and impedance grounded.